I. Field of the Invention
The present invention relates generally to data communication, and more specifically to techniques for transmitting pilot and signaling (e.g., rate control) information on the uplink in wireless communication systems.
II. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users sequentially or simultaneously by sharing the available system resources. Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
An OFDM system employs orthogonal frequency division multiplexing (OFDM) to effectively partition the overall system bandwidth into a number of (N) orthogonal subbands. These subbands are also referred to as tones, frequency bins, and frequency subchannels. Each subband may be viewed as an independent transmission channel that may be used to transmit data.
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors. For an OFDM system, the N subbands may experience different channel conditions and may achieve different signal-to-noise ratios (SNRs).
An accurate estimate of the response of the wireless channel between the transmitter and the receiver is normally needed in order to effectively transmit data on the available subbands. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. Since the pilot is made up of symbols that are known a priori by the receiver, the channel response can be estimated as the ratio of the received pilot symbols over the transmitted pilot symbols.
Pilot transmission represents overhead in a wireless communication system. Thus, it is desirable to minimize pilot transmission to the extent possible. However, because of noise and other artifacts in the wireless channel, a sufficient amount of pilot needs to be transmitted in order for the receiver to obtain a reasonably accurate estimate of the channel response. Moreover, because the contributions of the propagation paths to the channel response and the propagation paths themselves typically vary over time, the pilot transmission needs to be repeated. The time duration over which the wireless channel may be assumed to be relatively constant is often referred to as a channel coherence time. The repeated pilot transmissions need to be spaced significantly closer than the channel coherence time to maintain high system performance.
In the downlink of a wireless communication system, a single pilot transmission from an access point (or a base station) may be used by a number of terminals to estimate the response of the distinct channels from the access point to each of the terminals. In the uplink, the channel from each of the terminals to the access point typically needs to be estimated through separate pilot transmissions from each of the terminals.
Thus, for a wireless communication system, multiple terminals may each need to transmit a pilot on the uplink to an access point. Moreover, signaling information such as rate control information and acknowledgments for downlink transmission may need to be sent on the uplink. If the uplink transmissions are performed in a time division multiplexed (TDM) manner, then each terminal may be assigned a distinct time slot and would then transmit its pilot and signaling information in the assigned time slot. Depending on the number of active terminals and the duration of the time slots, a relatively large fraction of the uplink transmission time may be taken up by the pilot and signaling transmissions. This inefficiency in the uplink transmission of pilot and signaling information is exacerbated in an OFDM system where the data-carrying capacity of the smallest transmission unit (typically one OFDM symbol) may be quite large.
There is therefore a need in the art for techniques to transmit pilot and signaling information in a more efficient manner in wireless communication systems (e.g., OFDM systems).